This invention relates generally to turbine engines, and more particularly, to cooling systems for turbine engines.
A gas turbine engine typically includes a multi-stage axial compressor, a combustor, and a turbine. Airflow entering the compressor is compressed and directed to the combustor where it is mixed with fuel and ignited, producing hot combustion gases used to drive the turbine. As a result of the hot combustion gases entering the turbine, typically compressor air is channeled through a turbine cooling circuit and used to cool the turbine.
Compressor bleed air is often used as a source of cooling air for the turbine cooling circuit. However, extracting cooling air from the compressor may affect overall gas turbine engine performance. To minimize a reduction in engine performance, the cooling system may use fuel flowing through a heat exchanger to absorb heat from the compressor bleed air. As the fuel absorbs heat from the compressor bleed air, the temperature of the bleed air is lowered and engine cooling airflow requirements are reduced, thus reducing engine performance losses.
However, when fuel is heated, often carbon, gum, and coke deposits form within tubing used to transport fuel through the heat exchanger. Over time, such deposit agglomeration blocks individual tube passages resulting in higher fuel pressure losses in the heat exchanger and impaired heat transfer performance. Impaired heat transfer performance may result in less heat being absorbed from compressor bleed air and turbine components receiving compressor bleed air being cooled less effectively. As a result, such components are subjected to increased low cycle fatigue, LCF, stresses and increased thermal stresses. Furthermore, because turbine components are cooled less effectively, overall engine life is decreased.
In an exemplary embodiment, during gas turbine engine operations, a cooling system reduces fuel gum deposits within the cooling system and provides cooling air to the gas turbine engine. The cooling system includes a recirculating loop including a plurality of heat exchangers in fluid communication with the recirculating loop. A first heat exchanger is an air-fluid heat exchanger that uses heat transfer fluid to cool cooling air used by the gas turbine engine. A second heat exchanger is an air-fluid heat exchanger that uses engine fan air to cool the heat transfer fluid circulating in the recirculating loop. A third of the heat exchangers is a fluid-fuel heat exchanger that uses combustor main fuel flow as a heat sink to cool the heat transfer fluid circulating in the recirculating loop. The cooling system is selectively operable to reduce fuel gum deposits and provide cooling air to the gas turbine engine when the gas turbine engine is operating. A flow of heat transfer fluid through the third heat exchanger is modulated by a bypass line and valve to control the heat transfer from the heat transfer fluid to the fuel flow to reduce fuel gum deposits.
During gas turbine engine operations, the cooling system may be operated using either fuel flow or fan discharge airflow as a heat sink with all three heat exchangers in operation. Alternatively, the cooling system may be operated using fan discharge airflow as a heat sink with the third heat exchanger bypassed. Fuel is used as a heat sink when sufficient deposit dissipation forces are prevalent within the third heat exchanger or when a maximum temperature of the fuel can be controlled such that the temperature remains below temperatures conducive to fuel gum deposit formation. Fuel passing through the third heat exchanger flows through a path that increases heat transfer from the heat transfer fluid to the fuel. As a result of the flow through the fuel paths, the fuel flow develops high fluid turbulent forces and fluid shear forces that reduce fuel deposits within the third heat exchanger.